Installation of an oil or gas well involves fixing a tubular steel casing in cement in an underground bore. Holes must be subsequently created in the steel casing and cement in order to gain access to the surrounding formation, i.e., oil or gas deposit. Such holes are generally created through a process known as perforation. A well may also be required to be re-perforated from time to time if, for example, the flow of oil or gas becomes impeded by debris.
Perforating gun systems have been in use for or many years. A gun generally contains several explosive charges spaced from each other. There is a detonation cord running between and connected to the charges. The charges are generally arranged along the length of the gun to explode radially outwardly in different directions into the formation. A charge is contained in a case having an interior cavity shaped to deliver explosive forces to the desired degree of penetration into the formation and is thus referred to as a shaped charge.
Until a few years ago, "port plug guns" were commonly used and are still in use today, although less so. Such a gun includes a hollow tube having a wall thickness of between about 1.3 and 1.9 cm (1/2" and 3/4"), there being threaded holes or ports spaced along the tube. A shaped charge is inserted into the open end of the gun to the location of each port of the tube and secured in the tube by a port plug threaded into the port. After detonation, the gun is withdrawn from the well hole, the plugs and other remnants removed and the tube reloaded to be used again.
In order to increase charge density available for a particular gun, given the limits of the outer diameter of the gun, there has been a trend to systems having guns with thinner walls. The greater internal volume of gun tubes having thinner walls permits an increase in charge density. Thin-walled gun tubes, however, are less resistant to explosive forces and must be discarded after only one use.
It is not possible to thread the ports in thin walled tubes and threaded port plugs therefore are not used to secure charges in the thin walled tubes. At least two different approaches to holders for shaped charges have been patented. U.S. Pat. No. 4,598,775, of Vann et al., issued in July 1986 for Perforating Gun Charge Carrier Improvements. A charge holder is made up of a number smaller triangular holders connected together in tandem, each of the smaller holders being loaded with three shaped charges. U.S. Pat. No. 4,800,815, of Appledorn et al., issued in January 1989 for a Shaped Charge Carrier. In this case, a thin walled seamed tubular holder (referred to as a carrier in the specification of the Appledorn patent) has diametrically opposed tabs that are bent by means of a screwdriver down onto a surrounding upper lip of a shaped charge, diametrically opposed rear portions of the lip (at 90.degree. to the tabs) being supported by portions of the tube. The disclosures of these two issued United States patents are incorporated herein by reference.
At least as known to the inventor, the most common current approach to securing a shaped charge case in a thin walled tubular holder is to use a charge case that has a small leg extending downwardly from its base. The holder is manufactured with a small hole, diametrically opposite each larger port, through which the leg of the casing is inserted to protrude to the exterior of the holder. A clip is attached to the protruding leg to secure the casing in the charge holder and to hold the detonation cord in place. To be effective, the detonation cord has to be in appropriate contact with detonation material located in an aperture in the center of the base of the charge case.
This current approach has produced satisfactory performance in the field but the charge cases are machined and this is expensive,